Typical linear position sensors are described in an applications information publication entitled, “Linear Hall-Effect Sensors” by Allegro MicroSystems, Inc. of Worcester, Mass. Such linear position sensors are depicted in FIG. 1, for example, which is a drawing excerpted from the aforementioned publication. Each of these position sensors consists of a magnet field assembly 20 and a linear Hall-effect sensor 22. Magnetic field assembly 20 is comprised of two or four rectangular magnets 24 and soft magnetic housing 26. Linear Hall-effect sensor 22 crosses the magnetic field created by the magnets, in a left-right direction on the drawing sheet. When Hall-effect sensor 22 is located at the position D=0, the magnetic flux density is zero (0). This configuration is referred to as a “Push-Pull” approach in the aforementioned publication.
FIG. 2 illustrates a so-called “Push-Push” approach, where the Hall-effect sensor 28 moves between two magnets 30 of the same polarities. The movement is in a vertical (or up-down) direction on the drawing page. When the distance, D, corresponds to the midpoint between the two magnets 30, the magnetic flux density is zero (0). It should be mentioned that the magnets 30 in this case are also of a rectangular shape.
FIG. 3 depicts another approach: a bipolar slide-by mode. The illustration in FIG. 3 was excerpted from a publication entitled, “Hall Effect Sensing and Application, Micro Switch Sensing and Control,” by Honeywell. According to that publication, in the “bipolar slide-by mode” a linear Hall-effect sensor 32 moves relative to two rectangular magnets 34 attached to a magnetic plate 36 and separated from one another by a fixed distance. FIG. 3, illustrates how the magnetic field varies as a function of the distance 38 by which the center line 40 of the magnetic field assembly 42 is separated from the center 44 of the sensor 32.
All of the position sensors discussed above have pronounced non-linearity over their stroke range.
FIG. 10 is a reproduction of FIG. 2 from U.S. Pat. No. 6,215,299. As understood, in this configuration a single magnet is used to provide north and south polarities at the opposite ends of the magnet along the direction of motion. It is believed that with such a configuration, the disclosed sensor operates only on leakage flux from the magnet, which in turn affects the overall performance of the sensor. There is therefore a need for a position sensor in which the flux density versus stroke range can be better controlled, and in which improved linearity over the stroke range can be achieved.